The wet central heating system can be divided into two main categories which are the open vented system and the sealed system. Open vented system - Located in the loft are two tanks of difference sizes, a large cold-water storage tank feeds the hot-water storage cylinder and the other tank is a small feed-and-expansion tank, this vents the boiler, takes up any expansion in the system and tops up water losses from the system. The boiler has two jobs: The first is to supply heated water to the hot-water storage cylinder which heats the water supply serving the taps plus other outlets and the next job is to pump heated water to all the radiators. In older houses, water from the cold-water storage tank is fed to the hot-water storage cylinder by the force of gravity and the movement of water to the heating system is manage by a pump. The drawback to the gravity feed system is the low pressure from the taps and the gravity fed shower. This gets increasingly worse, the closer the outlet is to the cold-water storage tank and this is call the height differential. However this can be rectify by installing a pump. If the cold-water storage tank is above the outlet, it is call positive head but if the cold-water storage tank is below the outlet, it is call negative head.
Sealed system - This is similar to the open vented system but the difference being, there is no feed-and-expansion tank and this job is taken over by the boiler and pressure vessel. The boiler circuit is pressurize and mains water is top up as needed via a filling loop which has a non-return valve also a pressure vessel manages the expansion on the circuit during heating. Alternatively, the hot water cylinder can be unvented and fed mains-pressure water directly which means the cold-water storage tank is longer needed. If both the hot and cold water taps are supply with mains-pressure water, a pressure vessel and safety valve should be fitted to the hot water circuit. A combination boiler eliminates the need for a storage cylinder and hot water is fed from the taps via the boiler.